A magnetic assembly may include a bobbin having one or more windings wound around a central passageway through the bobbin. An inner core is positioned in the central passageway with first and second facing surfaces exposed at or near the two ends of the central passageway. The inner core has an inner core length between the two facing surfaces. An outer core has continuous outer walls that define an inner cavity. The outer core is positioned on the bobbin with the outer walls surrounding the bobbin and with first and second inner surfaces of the inner cavity juxtaposed with the first and second facing surfaces, respectively of the inner core. The outer core has a cavity length between the two inner surfaces. The inner core length and the cavity length are selected such that the cavity length is greater than the inner core length. A first gap is formed between the first facing surface of the inner core and the first inner surface of the inner cavity. A second gap is formed between the second facing surface of the inner core and the second inner surface of the inner cavity. The two gaps have a total gap width equal to the difference between the cavity length and the inner core length. To control the magnetic characteristics of magnetic assembly, it is desirable to control the widths of the two gaps.
The inner core and the outer core may be manufactured from a ceramic ferrite material based on iron-oxide, such as, for example, Mn—Zn ferrite, Ni—ZN ferrite or Mg—Zn ferrite. The ferrite material is formed into a desired size and shape by injection molding. One part of the forming process includes a sintering step at a high temperature (e.g., 1,280-1,380 C for a few hours). During the sintering process, the ferrite material may shrink up to 15 percent in any linear dimension, which may result in a loss of up to approximately 50 percent of the original volume of the original molded size and shape. Accordingly, the inner core and outer core are molded initially with sizes and shapes that are greater than a desired nominal shape to allow for the maximum expected shrinkage during the sintering process. If less than the maximum expected shrinkage of the inner core occurs, the inner core can be ground to the required inner core length. If less than the maximum shrinkage of the outer core occurs, the cavity length will be too short, and one or both of the inner surfaces of the cavity must be ground to provide the desired cavity length. The inner surfaces of the cavity are large compared to the size of each end of the inner core, and excessive grinding may be required to produce the desired cavity length.